RabbitMQ undefined: There is no template at js/tmpl/login.ejs

I had the same symptoms on my Windows server running RabbitMQ after shutting down the RabbitMQ service, and it refused to start back up.

Steps I took to resolve the issue without a reboot:

1. The RabbitMQ log file named rabbit@[SERVER_NAME_HERE].log at folder C:\Users\[ADMIN_ACCOUNT_USERNAME_HERE]\AppData\Roaming\RabbitMQ\log file was huge (12+ GB); I renamed it so that RabbitMQ could create a new log file.
2. I tried again to start the RabbitMQ service in services.msc, which failed (again).
3. The new rabbit@[SERVER_NAME_HERE].log file reported that a particular folder could not be deleted:

cannot_delete, "c:/Users/[ADMIN_ACCOUNT_USERNAME_HERE]/AppData/Roaming/RabbitMQ/db/rabbit@[SERVER_NAME_HERE]-plugins-expand/rabbitmq_management-3.3.5/priv/www/cli",

4. I tried to rename that cli folder in Windows Explorer and got a "That folder is in use by another process" error.
5. I downloaded the Sysinternals Handle.exe utility and ran it with an argument of "RabbitMQ", which showed that the folder (cli) that the log file reported couldn't be deleted was being locked by a rundll32.exe process with a particular process ID.
6. I killed that rundll32.exe instance in Task Manager (after turning on display of Process IDs on the View menu).
7. I started the RabbitMQ service again, and this time, it worked!

tl;dr

A particular Windows process was locking a folder that RabbitMQ wanted to delete as a part of its startup process. Identifying that process using the Sysinternals Handle utility and then killing it allowed RabbitMQ to start up successfully.

The problem was solved by restarting the Linux server as rabbitMQ commands were hanging and required force stop.

Hope this helps someone.